Advances in digital photography technology have led to an increase in the number of photographs being taken and stored in a digital format. In many instances, photographs taken with analog cameras are being processed and stored on digital storage media. Pictures are routinely scanned for storage as digital images. Many handheld devices, such as for example, personal digital assistants and cellular telephones, include cameras for capturing and storing digital photographs. Photographs are often shared via online digital photo albums. A user often has to sort through large numbers of digital photographs to select photographs for archiving and/or printing. In many cases, the decision to archive and/or print a photograph depends on the quality of the photograph. Users routinely discard photographs that are ill-exposed, in other words, images that are overexposed, underexposed or a combination of both where the image is overexposed in some sections and underexposed in other sections.
Many prior art devices determine whether a photograph or an image is ill-exposed based on an analysis of a luminance histogram of the distribution of the luminance level of the pixels in the digital image. One of more luminance parameters, such as for example, the average luminance of the pixels in the image, the standard deviation of the luminance of the pixels, and the distances between luminance peaks in the luminance histogram are evaluated to determine whether a digital image is an ill-exposed image.
Other prior art devices identify backlit images as ill-exposed images based on an analysis of the shape of a luminance histogram. A backlit image typically has some image areas that are overexposed and other image areas that are underexposed. Such prior art devices identify an image as an ill-exposed image if the luminance histogram is determined to have an inverse bell shape or two or more peaks. Yet another prior art device classifies an image as overexposed, underexposed, or normal based on an evaluation of the width, the center and the centroid of a luminance histogram of the image.
Prior art devices that identify ill-exposed images based on an evaluation of luminance histograms are typically unable to accurately evaluate whether predominantly black areas and predominantly white areas of an image are ill-exposed. For example, such prior art devices are typically unable to distinguish between an appropriately exposed predominantly black area of an image and an underexposed or overexposed predominantly black area of an image. Such prior art devices are also typically unable to distinguish between an appropriately exposed predominantly white area of an image and an underexposed or overexposed predominantly white area of an image.
Thus what is needed is a system and method of identifying ill-exposed images that seeks to overcome one or more of the challenges and/or obstacles described above.